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Atoms and Elements

Before we begin to learn about radiation, we must first develop a basic understanding of chemistry and physics.

Elements and the structure of the atom

All solids, liquids, gases, and plasmas (what scientists call matter, but you might simply call stuff) are ultimately made from atoms. You can think of atoms as the tiny building blocks that everything is made from. There are a limited variety of these blocks that exist (118 as of this writing), and we call these elements. You are probably already familiar with many of them (iron, lead, gold, silver, sodium, silicon, carbon, hydrogen, oxygen, nitrogen, etc.), and if you have ever taken chemistry you may know many more. You may also know of the periodic table of the elements, which is a convenient chart that arranges the elements according to their chemical properties.

There are two main parts of the atom: the nucleus and the electron cloud.


The nucleus forms the center of the atom, and contains nearly all of the mass of the atom. The nucleus is composed of two types of subatomic particles: protons and neutrons. Protons and neutrons both have approximately the same mass (neutrons are slightly more massive), but protons have a positive electric charge while neutrons have no charge at all. The protons and neutrons are held together by the strong nuclear force1, which is strong enough to overpower the electrostatic repulsion (two charges of the same type, like positive and positive or negative and negative, push each other away) between the protons.

The number of protons in the nucleus is called the atomic number, and it determines what kind of element the atom is. All atoms of the same element always have the same number of protons. However, different atoms of the same element can have a different number of neutrons in their nucleus. These alternative versions of the element’s nucleus are called isotopes. Different isotopes of the same element have different masses, but they all behave identically2 at the chemical level. However, as we will see later, the isotopes can have vastly different nuclear properties, and it is those differences which make much of nuclear science possible.

Because atoms can have different masses depending on the isotope, it is important to take note of the isotope’s mass (also called atomic mass). The atomic mass is the sum of the number of protons in the nucleus (given by the atomic number) and the number of neutrons in the nucleus. Fortunately, isotopes are usually named according to their atomic mass, so it is rare that you would actually have to do this sum on your own.

Above is a model of the nucleus depicting the protons and neutrons as colored spheres tightly packed into a group. It is important to note that nuclei in reality do NOT look or behave like this (due to quantum mechanics), but it is a useful mental model nonetheless.

Electron Cloud

The electron cloud, as its name suggests, is home to the atom’s electrons. Electrons are very small3, nearly massless, negatively charged particles which surround the nucleus. As shown in the figure above, the size of the electron cloud is much greater than the size of the nucleus. When we talk about the sizes of atoms, we are usually referring to the size of the electron cloud, not the nucleus. Because the nucleus is so small compared to the electron cloud, the vast majority (99.9999999999999%) of the volume of an atom is composed of empty space!

In a neutral atom, the number of electrons is equal to the number of protons in the nucleus. In this way, each element has a unique electronic structure, and it is the properties and behavior of the electrons in atoms that forms the basis of modern chemistry. However, it is not as important for the purposes of nuclear physics, so I probably won’t mention them very often in future lessons. It is the behavior of the protons and neutrons in the nucleus that we are interested in, and this will be the main focus of our brief introduction to nuclear physics in the lessons ahead.


  • Matter (stuff) is made of atoms (little building blocks).
  • Atoms are made of two parts: the nucleus (core) which contains the positively-charged protons and the neutral neutrons, and the electron cloud (shell) where the negatively-charged electrons live.
  • The number of protons in the nucleus determines the atom’s element (type), and this number is called the atomic number.
  • The number of protons plus the number of neutrons is the atomic mass (how “heavy” it is), and this number determines the atom’s element isotope (version of the element).
  • Isotopes of the same element almost always have the same chemical behavior, but generally have very different nuclear behaviors from one another.
  • In neutral atoms, the number of electrons is equal to the number of protons.
  • The behavior of the electrons controls the chemistry and the behavior of the nucleus (the protons and the neutrons) controls the nuclear properties. We will focus on the nuclear stuff and leave the electrons to the chemists!

Further resources

Crash Course Chemistry: The Nucleus

Khan Academy: Introduction to the atom

Chemwiki: The Atom

  1. Technically, it’s now called the residual nuclear force because we now know that it is the leftover force from the strong interaction between quarks inside the nucleons. 

  2. There are a few very subtle differences but they can usually only be observed under special conditions. 

  3. Electrons actually don’t have any size whatsoever to the best of our knowledge. Kind of weird to think about.